Dielectric resonators are used in many circuits for concentrating electric fields. They are commonly used as filters in high frequency wireless communication systems, such as satellite and cellular communication applications. They can be used to form oscillators, triplexers and other circuits, in addition to filters.
FIG. 1 is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. FIG. 2 is a perspective view of a microwave dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10. The resonators 10 are arranged in the cavity 22 of a conductive enclosure 24. The conductive enclosure 24 typically is rectangular. The enclosure 24 commonly is formed of aluminum and is silver-plated, but other materials also are well known. The resonators 10 may be attached to the floor of the enclosure, such as by an adhesive, but also may be suspended above the floor of the enclosure by a low-loss dielectric support, such as a post or rod.
Microwave energy is introduced into the cavity by an input coupler 28 coupled to an input energy source through a conductive medium, such as a coaxial cable. That energy is electromagnetically coupled between the input coupler and the first dielectric resonator. Coupling may be electric, magnetic or both. Conductive separating walls 32 separate the resonators from each other and block (partially or wholly) coupling between physically adjacent resonators 10. Particularly, irises 30 in walls 32 control the coupling between adjacent resonators 10. Walls without irises generally prevent any coupling between adjacent resonators separated by those walls. Walls with irises allow some coupling between adjacent resonators separated by those walls. By way of example, the dielectric resonators 10 in FIG. 2 electromagnetically couple to each other sequentially, i.e., the energy from input coupler 28 couples into resonator 10a, resonator 10a couples with the sequentially next resonator 10b through iris 30a, resonator 10b couples with the sequentially next resonator 10c through iris 30b, and so on until the energy is coupled from the sequentially last resonator 10d to the output coupler 40. Wall 32a, which does not have an iris, prevents the field of resonator 10a from coupling with physically adjacent, but not sequentially adjacent, resonator 10d on the other side of the wall 32a. Dielectric resonator circuits are known in which cross coupling between non-sequentially adjacent resonators is desirable and is, therefore, allowed and/or caused to occur. However, cross-coupling is not illustrated in the exemplary dielectric resonator filter circuit shown in FIG. 2.
An output coupler 40 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20. Signals also may be coupled into and out of a dielectric resonator circuit by other techniques, such as microstrips positioned on the bottom surface 44 of the enclosure 24 adjacent the resonators.
Generally, both the bandwidth and the center frequency of the filter must be set very precisely.
As part of the process of fine tuning such circuits, one or more metal plates 42 may be attached to a top cover plate (the top cover plate is not shown) generally coaxially with a corresponding resonator 10 to affect the field of the resonator in order to help set the center frequency of the filter. Particularly, plate 42 may be mounted on a screw 43 passing through a threaded hole in the top cover plate (not shown) of enclosure 24. The screw may be rotated to vary the spacing between the plate 42 and the resonator 10 to adjust the center frequency of the resonator.
In addition, tuning screws may be positioned in the irises between the adjacent resonators to affect the coupling between the resonators in order to tune the bandwidth of the filter.
The frequency and bandwidth of a dielectric resonator circuit depends on a great many factors. The sizes of the resonators 10, their relative spacing, the number of resonators, the size of the cavity 22, the sizes and positions of the tuning plates, the sizes and shapes of the irises 30, and the sizes, shapes, and positions of the tuning screws all need to be very precisely controlled to set the desired center wavelength and bandwidth of the filter.
As is well known in the art, dielectric resonators and dielectric resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell's equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is normally the transverse electric field mode, TE01 (or TE hereinafter). Typically, the fundamental TE mode is the desired mode of the circuit or system in which the resonator is incorporated. The second-lowest-frequency mode typically is the hybrid mode, H11 (or H11 hereinafter). The H11 mode is excited from the dielectric resonator, but a considerable amount of electric field lies outside of the resonator and, therefore, is strongly affected by the cavity. The H11 mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned (i.e., the enclosure) and has two polarizations. The H11 mode field is orthogonal to the TE mode field. Some dielectric resonator circuits are designed so that the H11 mode is the fundamental mode. For instance, in dual mode filters, in which there are two signals at different frequencies, it is known to utilize the two polarizations of the H11 mode for the two signals.
There are additional higher order modes, including the TM01 mode, but they are rarely, if ever, used and essentially constitute interference. Typically, all of the modes other than the TE mode (or H11 mode in filters that utilize that mode) are undesired and constitute interference.
The conventional techniques and mechanisms for tuning the frequency and/or bandwidth of dielectric resonator filters and other circuits have many shortcomings. For instance, the bandwidth of a dielectric resonator filter is a function of the field coupling between the individual dielectric resonators in the filter. The coupling between the dielectric resonators, and thus the bandwidth of the circuit, is primarily controlled by the size and shape of the irises between the resonators and the size and shape of the tuning screws positioned within the irises. The size and shape of the cavity also affects the bandwidth. Bandwidth tuning by adjusting the irises, tuning screws, and cavity is, largely, a process of trial and error and is tedious and labor-intensive and often consumes weeks. Particularly, each iteration of the trial and error process requires that the filter circuit be returned to a machine shop for re-machining of the cavity, irises, and/or tuning screws to new dimensions.
In addition, the tuning process involves very small and/or precise adjustments in the sizes and shapes of the irises, tuning screws and cavity. Thus, the machining process itself is expensive and error-prone.
Furthermore, the walls within which the irises are formed, the tuning screws and even the cavity all create losses to the system, decreasing the quality factor, Q, of the system and increasing the insertion loss of the system. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. The portions of the fields generated by the dielectric resonators that exist outside of the dielectric resonators touch all of the conductive components of the system, such as the enclosure 20, tuning plates 42, internal walls 32 and 34, and tuning screws 43, and inherently generate currents in those conductive elements. Field singularities exist at any sharp corners or edges of conductive components that exist in the electromagnetic fields of the filter. Any such singularities increase the insertion loss of the system, i.e., reduces the Q of the system. Thus, while the iris walls and tuning screws are necessary for tuning, they are the cause of loss of energy within the system.
Another disadvantage of the use of tuning screws within the irises is that such a technique does not permit significant changes in coupling strength between the dielectric resonators. Tuning screws typically provide tunability of not much more than 1 or 2 percent change in bandwidth in a typical communication application, where the bandwidth of the signal is commonly about 1 percent of the carrier frequency. For example, it is not uncommon in a wireless communication system to have a 20 MHz bandwidth signal carried on a 2000 MHz carrier. It would be very difficult using tuning screws to adjust the bandwidth of the signal to much greater than 21 or 22 MHz.
Even furthermore, it is difficult to implement cross-coupling between multiple dielectric resonators using the aforementioned conventional tuning techniques.
It is an object of the present invention to provide an improved dielectric resonator circuit.
It is another object of the present invention to provide a dielectric resonator filter circuit.
It is a further object of the present invention to provide improved mechanisms and techniques for tuning the center frequency of dielectric resonator circuits.
It is yet another object of the present invention to provide improved mechanisms and techniques for tuning the bandwidth of dielectric resonator circuits.